The ever-increasing importance of electronics in automobiles brings with it a growing challenge and need for low-cost, reliable electronic systems and subsystems that require input-output devices that interface with sensors and actuators. These systems and subsystems are not isolated, and must communicate with each other.
Historically, automotive electronics have been built up using discrete, smaller integrated circuits. They relied on proprietary, dedicated wire communication schemes, at least for many sensor systems, and directly wired power outputs to the actuators. This led to large printed-circuit boards (PCBs), large engine-control unit (ECU) housing sizes, and excessive wiring bundles. Wiring brings with it other problems since it consumes space, adds weight and expense, is subject to the vehicle's electromagnetic noise, and can be difficult to trouble shoot and maintain.
Fortunately, advances in vehicle-networking standards and mixed-signal semiconductor processes are addressing these issues and introducing new possibilities to distribute intelligent systems throughout a vehicle. The trend in vehicle-networking standardization includes the wide adoption of Controller Area Network (CAN) and the Local Interconnect Network (LIN) architecture, now in version 2.1.
These network standards are providing a balance between performance and cost optimization across automotive systems. CAN provides a high-speed network for chassis, power-train and body-backbone communications, while LIN answers the need for a simple network for sensor and actuator subsystems that reduces cost and improves robustness through standardization. The wide use of CAN and the availability of LIN coincides with advances in mixed-signal semiconductor-process technologies that can bring together all the functionality needed for smaller automotive systems onto a single integrated circuit (IC), or a few ICs for more advanced systems.
While LIN was originally targeted for the vehicle's body electronics, it is proving its value in new ways with many implementations outside of body electronics. Among the automotive-electronic bus standards available, LIN provides the best solution for the communication needs of most sensors and actuators which are normally dedicated to a single system. They can be viewed as subsystems and are well served by LIN, which has been defined to fill a sub-network role in the vehicle. The maximum LIN specified data rate of twenty kilobits per second (kbps) is sufficient for most sensors and actuators. LIN is a time-triggered, master-slave network, eliminating the need for arbitration among simultaneously reporting devices. It is implemented using a single wire communications bus, which reduces wiring and harness requirements and thus helps save weight, space and cost.
Defined specifically for low-cost implementation of vehicle sub-network applications by the LIN Consortium, the LIN standard aligns well to the integration capabilities of today's mixed-signal semiconductor processes. The LIN protocol achieves significant cost reduction since it is fairly simple and operates via an asynchronous serial interface (UART/SCI), and the slave nodes are self-synchronizing and can use an on-chip RC oscillator instead of crystals or ceramic resonators. As a result, silicon implementation is inexpensive, making LIN very suitable for the mixed-signal process technologies typically used to manufacture signal-conditioning and output ICs for automotive subsystems.
The LIN master node is normally a bridge node of the LIN sub-network to a CAN network, and each vehicle will typically have several LIN sub-networks. The master LIN node has higher complexity and control, while the slave LIN nodes are typically simpler, enabling their integration in single IC subsystems. Through the use of standardized vehicle-networking architectures, it is possible to build a feature- and diagnostic-rich system that requires only three wires (LIN, battery and ground)
For obvious reasons of reliability and safe operation a very high immunity for both ESD (Electro Static Discharge) and DPI (Direct Power Injection) is required for all the LIN modules. This high ESD and DPI immunity specially applies to the pins of a LIN module that are connected to the external world (e.g., battery pin, LIN pin, etc.)
The pins of a LIN module that are connected to the system (external world) are highly exposed to ESD discharge when the module is handled or plugged into the system. A LIN module must be able to be safely installed or removed by any one. Therefore the ESD immunity needs to be very high (greater than several kilovolts) for all of the LIN module pins since the standard industry rules for handling an electrical module cannot be properly enforced in the automotive industry.
In addition once installed, any pin connected to the LIN system may see a high level of interferences coming from the other communications busses and/or power supply lines. The reason is that the communications busses and power supply lines cannot be wired with efficient shielding or differential signal lines (except for CAN) for cost reasons. Therefore the high interference levels present in automobile electrical and control systems must not impact the integrity of the desired data transiting on the LIN bus.
Thus very high immunity to both ESD and DPI is required for any pin of a chip that is directly routed to the connector of a LIN module. A commonly used device for ESD protection is a grounded gate metal oxide semiconductor (GGMOS) transistor that has its gate grounded through an ESD protection resistor. A common technique to enhance the ESD robustness of the GGMOS transistor used for ESD protection of a respective external connection (pin) is to have some capacitive coupling between the drain and the gate of the GGMOS transistor protection device.
Unfortunately this ESD protection technique dramatically increases the sensitivity of the pin to noise interferences or DPI: The capacitive coupling between the drain and the gate allows high frequencies to reach the gate of the protection device and turn it on. This corrupts the desired data flow. Therefore the capacitive coupling significantly degrades a high DPI robustness. Therefore, standard ESD protection techniques are not well suited to achieve a high noise and interference immunity for DPI and the like.